In industrial installations, low-voltage loads, such as e.g. control circuits, amplifiers and similar, are supplied with a DC voltage which is non-hazardous to persons, preferably of 24 V. Appropriate power supply devices for the supply of such a DC voltage can deliver output currents of 20 A or more. At such high currents, protective devices, such as e.g. fuses or circuit-breakers, must be connected in series with the respective loads, in order to protect the latter, and specifically the supply conductors thereof, against thermal overloads or short-circuit currents. In order to permit the secure magnetic tripping of circuit-breakers upon the occurrence of an electrical fault, for example a short-circuit, tripping currents are required which are approximately 7.5 times the rated current specified for the power supply device. The trip behavior of circuit-breakers is defined by the time/current tripping characteristic thereof, for example the B characteristic. With customary dimensioning, conventional 50 Hz transformers which are employed as power supply devices can, in the event of a short-circuit, deliver high tripping currents of this type for circuit-breakers. On the grounds of high electrical losses and the heavy weight, 50 Hz transformers of this type in industrial power supplies are increasingly being replaced by electronic power supply devices, such as e.g. switched-mode power supplies with a high switching frequency. However, upon the occurrence of an electrical disturbance, electronic power supply devices customarily restrict the output current very rapidly, i.e. between 10 and 100 μsec, to between 1.1 and 1.5 times the rated current value, in order to protect loads and supply conductors against thermal overloads and short-circuit currents. Accordingly, the secure tripping of an electromagnetic protection device cannot always be guaranteed.
An increased output-side energy demand is not only associated with a fault, such as the aforementioned short-circuit. The switching-in of loads, for example the start-up of an electric motor, can also result in a short-term increase in energy demand on the output side of the power supply device. In this case, reference would generally be made to a special condition involving increased energy demand. A malfunction, such as the aforementioned short-circuit, is an extreme manifestation of such a special condition.
Upon the switching-in of a large capacitive load, the output voltage dips rapidly, then rises again, as a function of the loop impedance and capacitance. Upon the switching-in of an ohmic-inductive load, for example a motor, a high start-up current flows, which is only limited by the ohmic resistance of the motor winding. Customarily, normal operation is restored within a few seconds. During the transition time, however, the load can draw output currents from the power supply device which are substantially in excess of 1.5 times, but lower than the value of 6 to 7.5 times the rated current value which is required for the tripping of the protective device, and for a variable and load-dependent time interval which cannot be predetermined.
From DE 10,2005,031,833 A1, a method is known for supplying energy to a low-voltage load which is protected by a protective device, by means of an electronic power supply device—a switched-mode power supply—whereby the input and/or output voltage of the power supply device is monitored, in order to detect a drop in the input and/or output voltage below a threshold. Upon the detection of a drop in the input and/or output voltage below the threshold, a current is delivered for a predetermined time interval, the magnitude of which is rated for the secure tripping of the protective device and, upon the expiry of the predetermined time interval, the current is limited to a lower value. According to the prior art, a drop in the input or output voltage below a threshold is interpreted as an indication of a malfunction or a special situation and, and in response thereto, an increased output current is delivered, which is sufficiently high to ensure the secure tripping of a protective device. In this arrangement, a response of the power supply device which is adapted to a specific type of malfunction or special condition is not possible. The prior art does not distinguish between a special condition which involves, for example, a short-circuit or a motor start-up.